138 ON COMETS. 



form of existence, in which, like the air itself, they are 

 invisible. As the comet then gets heated a portion is 

 actually vaporized and the vapour condenses as it 

 cools again. The whole substance of the comet of 

 Halley, as you have heard, was so evaporated in 1835-6, 

 all but what I suppose must have been really its solid 

 body ; that star which I have already mentioned, which 

 was seen on the 22cl January 1836 : and all that curious 

 process that went on afterwards, no doubt was that of the 

 re-condensation of the evaporated matter, and its gradual 

 re-absorption into and close around the body. 



(52.) There is still one point in the history of comets 

 which I have not touched upon, or but slightly. Compa- 

 ratively only a few of the great number of comets which 

 have been observed, and of which the orbits have been 

 calculated, have been seen more than once the great 

 majority once seen, seem lost for ever. What becomes 

 of them, is a very natural question. The answer to this 

 is, that the time of the periodical return of a comet 

 depends entirely on the distance to which it may run out 

 from the sun. Now we know of nothing to interfere 

 with or disturb the motion of a comet, once clear of the 

 planetary system, between the farthest planet and the 

 nearest fixed star ; and that interval is so immense that 

 the imagination is lost in attempting to conceive it. The 

 farthest planet we know of is only 30 times the distance 

 of the earth from the sun. Halley's comet in its ellip- 

 tic orbit of 75 years, goes only a little beyond that, or to 

 about 36 times the earth's distance. Donati's comet, if 

 the computists are right, will return in 2100 years, and 



